Carbonates such as ethylene carbonate, propylene carbonate and dimethyl carbonate are generally used as a solvent for dissolving an electrolyte salt to be used for a non-aqueous electrolytic solution of a lithium secondary cell. However, these hydrocarbon carbonates are low in flash point and high in combustibility, and accordingly, especially in large size lithium secondary cells for hybrid cars and distributed power source, enhancement of incombustibility of a non-aqueous electrolytic solution is an important problem to be solved for securing safety.
In order to enhance incombustibility (flame retardancy) without lowering performance of a non-aqueous electrolytic solution, addition of a fluorine-containing solvent is proposed (Patent Documents 1 to 6).
However, in these Patent Documents 1 to 6, there is a problem that solubility of an electrolyte salt is low, and LiPF6 and LiBF4 which are excellent electrolyte salts and are generally used cannot be dissolved and that viscosity is high, thereby lowering rate characteristic.
As mentioned above, the present invention is such that non-aqueous electrolytic solutions assuring good solubility of an electrolyte salt and having incombustibility (flame retardancy) and enough cell characteristics (charge and discharge cycle characteristic, discharge capacity, etc.) have not yet been developed.